This invention relates to a lithium oxyhalide cell or primary battery system with a cathode catalyst that provides the system with a significantly improved discharge rate and discharge capacity.
The recent growth in portable electronic products has produced a demand for light electrochemical power sources with a high rate of discharge and a high energy storage capacity. This demand has created much interest in lithium cells and batteries. A lithium cell refers to an electrochemical cell using a highly reactive, yet light, lithium anode, in combination with various cathode and electrolyte materials. The interest in this cell technology stems from a recognition that such cells can theoretically provide higher energy densities and higher voltages than conventional cells.
Lithium cells consist of a light, highly reactive anode containing lithium, an active cathode depolarizer, an ionically conductive electrolyte, and a cathode current collector. Liquid oxyhalides of an element of Group V or Group VI of the Periodic Table function as both active cathode depolarizers and electrolyte solvents. Oxyhalides such as phosphoryl chloride, vanadyl trichloride, vanadyl tribromide, thionyl bromide, thionyl chloride, sulfuryl chloride, pyrosulfuryl chloride, chromyl chloride, selenium oxychloride, and selenium oxyfluoride when included in cells, result in cells with high energy density and current delivery capability, especially in conjunction with a stable lithium complex such as lithium aluminum tetrachloride.
It should be understood that of the foregoing list of oxyhalides used in lithium cells, lithium-thionyl chloride, lithium-sulfuryl chloride and lithium-phosphorous chloride have been employed more frequently in commercial applications. Thus, when reference is made to either lithium-thionyl chloride, lithium-sulfuryl chloride, or lithium-phosphorous chloride, it is to be understood that any other liquid "oxyhalide" is an equivalent.
Prior lithium oxyhalide batteries suffer from poor long term storage capability following high rate discharge and from a lack of safety. During high rate discharge, unstable chemical reaction species are formed at the anode and reaction products such as lithium chloride are deposited on the cathode. As the internal resistance rises, overheating occurs and there is a risk of explosion.
Methods to increase the discharge rate capability and decrease formation of reaction products on the electrodes include the addition of a phthalocyanine complex to the cathode, use of a corroding cathode, and addition of AlCl.sub.3 and butyl pyridinium chloride to the electrolyte solution.
U.S. Pat. No. 4,252,875 entitled Electro-Catalysts For the Cathode(s) To Enhance Its Activity to Reduce SOCl.sub.2 in Li/SOCl.sub.2 Battery by Hanumanthiya V. Venkatasetty teaches improving the discharge rate of oxyhalide systems by applying a coating of phthalocyanine complex on the cathode. However, these complexes tend to dissolve into the electrolyte thereby reducing their usefulness.
U.S. Pat. No. 4,366,212 entitled Non-Aqueous Cell With Fluid Cathode Depolarizer and Corrodible Cathode by Dey et al. teaches that use of a cathode, corrodable by the electrolyte solvent at the same rate at which deposits are made onto the cathode, prevents a decrease in discharge rate. The disadvantage is that the cathode and the fluid cathode depolarizer/electrolyte solvent must be maintained separately until the battery is used since the cathode degrades following exposure to the solution.
U.S. Pat. No. 4,355,086 entitled Lithium Thionyl Chloride Battery by Saathoff et al teaches improving the discharge rate and decreasing the chance of overheating by increasing the conductivity of the electrolyte solvent through the addition of highly conductive acidic salt mixtures. This scheme does not eliminate the problem of deposition of lithium chloride on the cathode with subsequent loss of discharge rate capacity.
U.S. Pat. No. 4,296,185, discusses a possible solution to long term storage of lithium oxyhalide batteries. This patent is based on an alternative theory that the build-up of compounds on the anode causes a decrease in current rate delivery after storage ("passivation").
It is therefore apparent that lithium oxyhalide systems would be particularly suitable for many applications if the rate of discharge and discharge capacity could be improved.